This invention relates to biaxially oriented films, particularly to inline biaxially oriented polyester films having a thermally stable, releasable polymeric coating.
It is desirable to provide a coated polyester film for the production of optically variable pigments. The coating for use in this process is preferred to be processed inline to provide for the lowest cost of production. Furthermore, this coating should have the following unique and special attributes:
1) High thermal stability,
2) Very rapid dissolution into a recovery solvent such as acetone,
3) Water insolubility,
4) Tg greater than 0C, Tg less than 50C as defined by the differential scanning calorimetery (DSC) method,
5) Highly extensible to survive an inline coating process, without turning hazy or white,
6) Substantially noncrosslinked,
7) Surface Energy  less than 40 dyne/cm and  greater than =35 dyne/cm.
Others have attempted to produce polyester films of the type described above, but have had limited success. For example, U.S. Pat. No. 5,968,664 discloses a polyester film with a releasable coating comprising copolymers of methylmethacrylate (MMA) and ethylacrylate (EA) and homopolymers of polyacrylic acid (AA), wherein the coating layers are soluble in the release solvent. However, no mention is made concerning the limitations of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures.
U.S. Pat. No.5,795,649 discloses a polymeric coated film including water soluble copolymer of styrene (S) and an alpha, beta-unsaturated carboxylic acid or anhydride like maleic anhydride, wherein the molecular weight of the copolymer is from about 700 to 10,000. However, the molecular weight restriction limits the actual processability of the materials. Also, there is no mention of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures.
U.S. Pat. No. 5,928,781 discloses a method of making a thin layer or flakes of materials by using a coated substrate. The coating is defined as solvent soluble and containing a Crotonic acid polymer. Again, the important limitations of thermal stability, quantification of acetone solubility, metal release, surface energy and glass transition temperatures are not mentioned.
U.S. Pat. No. 6,013,370 discloses a method for making optically variable pigments by use of a sputtering process. The mode of cracking of the flake is indicated as being a key to the process. It discloses that the coating assists in producing flake of high aspect ratio, and mentions it as desirable.
This invention relates to a biaxially oriented polyester film having a substantially non-crosslinked polymeric coating containing styrene and acrylate, wherein the coating is highly thermally stable with a primary onset temperature of greater than about 350xc2x0 C., having a glass transition temperature between about 0xc2x0 C. and about 50xc2x0 C., having a solubility in a low molecular weight organic solvent, and having a surface energy of greater than about 35 dyne/cm and less than about 40 dyne/cm.
Thermal stability in accordance with this invention is defined herein as the temperature at which a 10% weight loss occurs of the primary decomposition curve for the polymeric coating materials. In determining thermal stability, the coating material is heated in a nitrogen atmosphere from ambient to 105xc2x0 C. at 10xc2x0 C./min. The sample is then held at 105xc2x0 C. for 15 minutes then ramped to 550xc2x0 C. at 10xc2x0 C. per minute. The temperature corresponding to 10% weight loss is read from the weight loss curve.
Solubility was measured by immersing the coated film into a bath of acetone for approximately 2 seconds. The films were then removed and allowed to air dry. The films were then inspected with a light microscope at 40xc3x97magnification and grouped by percentage of coating removal. Thus, rapid solubility is defined when at least about 75% of the coating is removed by a low mol weight solvent, preferably acetone.
Surface energy is defined from measurements of the contact angle of water on the surface of the coated PET base film. The surface energy numbers reported here are derived from conversion of the contact angle of water on the surface to absolute wetting tension values.
Glass transition temperatures (Tg) are reported from manufacturers literature. They are obtained by making measurements by use of the well-known DSC method. Glass transition temperatures were measured by the manufacturers of the polymer emulsions via the use of differential scanning calorimetery (DSC). DSC is a well-known method in the art to characterize thermal phase transitions of polymeric materials; please see Allcock, H. R., Camp, J. W. xe2x80x9cContemporary Polymer Chemistryxe2x80x9d, Prentice-Hall 1990, Chapter 17, which describes DSC as the Industry Standard technique for measuring Tg values.
Metal release is measured from bell jar metallizing the coated PET or related polymeric samples with aluminum metal to an OD of about 2.5. The metallized samples are then submersed in acetone for approximately 2 seconds, removed, and then brushed gently with a dry cloth. A metal release value of Excellent means that essentially all aluminum was released when using this technique. A metal release rating of fair means that approximately 25-50% of the metal was released via this technique. A metal release value of poor means that less than approximately 25% was released by this technique.
Crosslinking is the process whereby adjacent polymer chains are covalently linked, ultimately increasing the molecular weight of the polymer materials. If sufficiently high crosslinking density is achieved, the bulk material can be considered a single molecule in that the individual polymer molecules have been linked through one or more covalent linkage points with another polymer chain. During increasing crosslinking density of the material, changes to the melt flow, Tg, decomposition temperature, solubility in various solvents and other physical and chemical changes to the polymer develop. However, at low, or as often defined in the art, xe2x80x9clightxe2x80x9d, crosslinking density, the polymer material acts essentially or substantially as a non-crosslinked material. Changes that significantly or materially impact the chemical or physical attributes of the polymeric material are, therefore, defined as significant or substantial crosslinking and are undesirable in the context of the invention. The degree of crosslinking of the coating should preferably be kept below about 1% by weight, based on the weight of the polymer. Crosslinking in an amount less than about 1% by weight is clearly substantially non-crosslinked.
Although not wishing to be bound by any particular theory, we believe the following description of the physical process employed is helpful in illustrating the invention.
In the vacuum sputtering process of polyester film (PET), an organic surface (the PET film) is in contact with a hot metal gas (the sputerant). The temperature of this gas may approach 1000C. When the hot metal gas contacts the organic coating, decomposition of the underlying substrate often occurs. This decomposition may result in crosslinking of the coating, shrinkage of the coating or off gassing of decomposed adducts. Crosslinking of the coating will render it insoluble and, thus, unable to release the inorganic pigment. Thermal shrinkage or off gassing of decomposition adducts degrades the metal layer through increased cracking and pitting of the metal layer. Therefore, it is desired to construct a coating surface having high thermal stability to resist the effects of high thermal loads from the sputtering process.
Concurrently, metal having a high surface energy has stronger adhesion to surfaces having high surface energy. An increase in metal/coating adhesion forces increases the time required to release the sputter coated metal from the surface of the coated PET. Therefore, we have found that for best processing of the metallized films, that a surface energy SE between about 35 dyne/cm and about 40 dyne/cm is preferred.